I am still curious about what happened. It seemed that he took every precaution (except actually looking into the chamber and lower reciver). Still, with dropping the mag and clearing the chamber, this should not have happened.
No. He didn't really check. Looks like he might have chambered a round from the mag?
The video is pretty blurry to make it out, but it sure seems like the guy spent alot of time mugging for the class and not paying attention to what he was doing. There's a chance the weapon was defective in some way, but more likely than not, he pulled the trigger on a chambered round.